The present invention relates to electronic circuits and systems, and more particularly, to electronic circuits and systems including amplifier circuits and methods of operating the amplifier circuits.
Amplifier circuits and methods find many applications in electronic systems. For example, amplifier circuits are widely used in wireless communication devices such as pagers, cellular phones, or cellular base stations to process various analog signals within the system. The function of an amplifier is to increase the power, voltage, or current of signals received at the amplifiers input or inputs. One common application of an amplifier circuit is for transmission of a Radio Frequency (RF) signal. For example, many wireless communication systems require a power amplifier to drive the antenna of the system, thereby transmitting information contained in the amplified signals into the airwaves.
FIG. 1 illustrates a conventional MOSFET RF amplifier 100 for amplifying the power of an RF signal. The amplifier circuit 100 includes an input matching network 110, a MOS transistor 120, a load 130, a bias network 140, and an output matching network 150. The bias network includes a pair of resistors 141 and 142 connected in series between a fixed bias voltage +VB and the gate of MOS transistor 120. The first terminal of a capacitor 143 is connected to the node between resistors 141 and 142, and the second terminal of the capacitor 143 is connected to ground.
FIG. 2 illustrates a conventional N-channel MOS transistor structure 200 that may be used in the amplifier circuit of FIG. 1. MOS transistor 200 includes a body region comprising a P-type substrate 210, a N-type source region 221, a gate 230, and an N-type drain region comprising N− region 242 and N+ region 241. The drain, source, and body regions include electrical contacts 240, 220, and 250, respectively. In typical amplifier designs using MOS or equivalent devices, the body is electrically connected to the source. A common technique for establishing the source to body connection is a polysilicon connection 260 between the source contact 220 and body contact 250. Thus, in typical amplifier designs, the body is maintained at the same voltage as the source.
The circuit shown in FIG. 1 is representative of a single MOSFET amplifier stage. This can be considered a single ended stage or half of a balanced stage. Load 130 may comprise an inductor, shown as jωL1, which may have a high impedance at the circuit's frequency of operation. As is the case with inductors, any sudden change in current will cause a voltage of opposite polarity to be generated. The magnitude of this voltage will be proportional to di/dt.
Also shown in FIG. 1 is an input signal 101. Input signal 101 includes a first amplitude signal portion defining a first signal envelope 102, and a brief large amplitude pulse portion defining a second signal envelope 103. Such signals may be common in a variety of electronic systems. For example, in a wireless communication system, such signals may occur when the various wavelengths in an encoded signal constructively interfere. Signals including brief pulse envelopes are problematic to amplifiers because, typically, the amplifier is biased to handle the smaller envelope 102. Thus, the bias current drawn by amplifier circuit 100 will be primarily determined by the smaller constant amplitude signal. However, when a large amplitude pulse is received, there will be a definite di/dt present across the inductor. The magnitude of the di/dt will depend on the slope of the large amplitude pulse. A large slope will typically cause a large voltage to appear across the inductor, and thus, the output of the amplifier will clip at the supply voltage. Accordingly, any information carried in the input signal 101 may be lost or severely distorted.
Another problem with conventional amplifier circuits and methods is that various electronic system applications, such as wireless communication systems, for example, have an ever increasing requirement that the amplifiers include more functionality and improved performance. For example, electronic systems may require amplifier circuits or methods that have variable gains, wide bandwidths, process or temperature compensation, improved linearity, or power efficiency.
Accordingly, amplifier circuits and methods that have improved performance and increased functionality are desirable for modem electronic systems.